A promising absorber material for use in thin-layer solar cells is the material system of the chalcopyrite semiconductors I-IIII-VI2 with the currently most widespread representative CuInSe2 and its alloy Cu(In,Ga)Se2. A current construction for thin layer solar cells based on this absorber material consists of a glass substrate on which a metallic back contact of molybdenum is applied; the chalcopyrite absorber layer follows in a thickness of customarily 1-3 μm, a cadmium sulfide buffer layer on it and finally a transparent, conductive front contact, for example, of aluminum-doped zinc oxide. Such solar cells achieve efficiencies of up to 19.9% in the laboratory.
Instead of the rigid glass substrate, flexible substrates of metal sheets or polymer sheets have proven themselves. Independently of the selection of the substrate, only substrate will be used in the following. The illustration 1 on page 616 in can serve as an example for the construction of a flexible Cu(In,Ga)Se2 solar cell on a polyimide sheet.
Various methods are known for the deposition of the Cu(In,Ga)Se2 absorber layer. For example, the metallic components copper, indium and gallium are made available in a sequential process by previously deposited precursor layers. This layer stack is subsequently chalcogenized by being rapidly heated up in an atmosphere of selenium. During the simultaneous deposition by a co-evaporation the metallic components as well as the chalcogen component (selenium or sulfur) are simultaneously deposited. In addition, a deep element gradient can be produced in the absorber layer by a purposeful arrangement of the metallic evaporators which raises the efficiency of the thin-layer solar cell.
As a rule the substrate is heated in the deposition processes.
Furthermore, it is known in the simultaneous or sequential deposition of Cu(In,Ga)Se2 as absorber material for thin-layer solar cells that the selenium component can be made available by an ion beam. For this, on the one hand the selenium component is made available in a more reactive form than it would be in the case of evaporation of pure selenium. On the other hand an additional, non-thermal energy contribution for layer growth is made available by the energetic selenium ions. This has a positive influence on the layer growth and results in a higher quality of the polycrystalline absorber with a simultaneously reduced substrate temperature.
Furthermore, it is known that in the deposition of Cu(In,Ga)Se2 only gallium can be offered in ionized form. This has a positive influence on the layer growth and the qualities of the absorber layer. The gallium ions originate from a gallium ion beam source.
Furthermore, solid ion beam sources are known that can generate ions for research purposes from materials with a high evaporation point (e.g., copper and indium). The rate (i.e. evaporation rate) as well as the ion current density of these sources are very low and therefore not suitable for the layer deposition on an industrial scale.
Furthermore, a method is known that contains the co-evaporation of copper, indium, gallium and selenium. In the resulting vapor phase the elements are mixed, whereby during the evaporation in the space between the evaporator sources and the substrate a plasma is ignited and maintained. This results in an ionization and excitation of all layer-forming elements. In the production of Cu(In,Ga)Se2 absorber layers by co-evaporation, according to the state of the art as a rule pure metallic and chalcogenic vapors are used in a vacuum for the introduction of Cu, In, Ga and Se into the absorber layer. The latter have a lower reactivity in the Cu(In,Ga)Se2 formation of the absorber layer than excited and/or ionized Cu, In, Ga— or Se components. The layer growth and therewith also the process of crystallite formation are not additionally energetically influenced. For this reason an additional, conventional thermal heating of the substrate during the deposition is necessary. As a rule, temperatures up to 550° C. are used for the recrystallization of the absorber. Therefore, on the one hand a deposition on temperature-sensitive substrates (such as, for example, polyimide sheet), that can therefore only be heated to a low extent is not possible or only possible with a considerably lesser efficiency of the solar cells. On the other hand, high energy costs accumulate during the production of the Cu(In,Ga)Se2 absorber layers on account of the high substrate temperatures.
The simultaneous excitation and/or ionization of all layer-forming components (Cu, In, Ga, Se) in the coating chamber by means of an additional plasma excitation can only partially solve this problem. As the reactivity of the layer-forming elements is elevated, however, an additional, non-thermal energy contribution into the growing Cu(In,Ga)Se2 layer cannot take place on account of the low energy of the ionized and/or excited particles of only a few eV i.e., high substrate temperatures continue to be required for a sufficiently good crystal quality. A further disadvantage of this process is the fact that as a result of the homogenous mixing of the metallic vapors and chalcogenic vapors the adjusting of a deep distribution of the elements, that is advantageous for the electrical qualities of the absorber layer, is not possible. Furthermore, no purposeful deposition on a substrate but, additionally, even on the inner wall of the coating chamber takes place, which elevates the consumption of material and therefore the material expenses in the production of economical thin-layer solar cells.
The solid ion beam sources known from the state of the art would make possible an ionization of all metallic components and chalcogenic components; however, these ion sources are very expensive and an individual ion source would have to be used for each component. The use of these additional ion beam sources for the individual metallic elements consequently has a very high cost for investment and control. This results in a distinct rise of the production costs. Furthermore, these sources do not make possible a deposition of large-area absorber layers since on the one hand their exit opening is as a rule limited to a diameter <10 cm and on the other hand the solid ion sources have too small a density of the ion current and they can therefore not make available amounts of material that are necessary for an economical production. Furthermore, the ion energies that can be adjusted with these sources are also clearly above the ion energies required for the absorber deposition process. The lowering would mean additional expenditures.
The use of an individual broad-beam ion source for the chalcogen component does allow an additional non-thermal making available of energy in the growing layer and therefore the use of lower substrate temperatures; however, the metallic components (Cu, In and/or Ga) continue to be offered in their non-reactive form. This reduces the growth rate of the absorber layer.